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The Romans prayed to them. They helped usher in the Iron Age. A big one wiped out the dinosaurs. And we are being showered by them. Meteors. Where do these shooting stars come from? What are they made of? It turns out that we don’t have to worry about meteor showers, even though the sky really is falling.

Let's start with some definitions. A meteoroid is a chunk of space rock or dust when it’s still flying around out in space. When it hits our atmosphere, it does two things. It changes names and becomes a meteor, and it lights up from friction with the air as it flies at speeds of between 7 and 46 miles per second — 25,200 to 165,600 miles per hour!

If it’s big and bright enough, the meteor is defined as a fireball. Scores of fireballs are sighted every year. If the fireball explodes in flight, it’s called a bolide. If chunks of a meteor, fireball or bolide fall to the ground, they are called meteorites.

Space is full of dust and chunks of rocks. And the earth is being hit by this stuff all the time. An estimated 25 million meteors fly through the atmosphere every day. That adds up to a few tons of stuff — but most of the pieces are literally the size of specks of dust or small pebbles and burn up within seconds of hitting the atmosphere. Several thousand meteors each year are big enough to be categorized as fireballs.

Fortunately, 95 percent of all meteors, big or small, are made up of the debris spewed out by the tails of comets. These materials, dust and ice and rocks, are too fragile to survive their fiery flight. So they never survive to become meteorites. They never land. We don’t see most of them because most occur over the ocean, many occur during the day, and even the ones that occur at night flash over us while we’re asleep or indoors. You know what they say: “If a meteor falls at night and there’s no one there to see it, did it really fall?”

All the meteors in meteor showers are comet-tail dust. Meteor showers occur during the few days when the orbit of the earth around the sun intersects with the stream of debris strung out for millions of miles behind a comet in its orbit around the sun. On an average night, you might see as many as eight shooting stars. During a good meteor shower, you might see as many as 60 or more per hour.

By the way, while you’re watching the fiery show, you could be seeing the original source of water on earth. Some scientists think that the vaporization of comet ice in meteors is where water on this planet first came from.

Name and colors
Meteor showers get their names from the constellation nearest to where it looks like most of the meteors are coming from. In fact, that’s a good tip for meteor-shower viewing. If you want to watch for the Perseids — debris from the tail of Comet Swift-Tuttle — look to the part of the sky near the constellation Perseus in mid-August.

The Leonid meteor shower seems to emanate from near the constellation Leo in mid-November. The source of that shower is the trail left behind by Comet Temple-Tuttle (Horace Tuttle was a big comet spotter in the 1800s).

Meteors flash different colors — white, blue, red, green, orange, yellow — mostly depending on what they’re made of. Different minerals emit different wavelengths of visible light when they burn. That’s the same reason why different minerals are used to give fireworks their colors. Shooting-star colors also depend on the speed of the meteor, and the angle at which it hits the atmosphere.

While meteors are mostly comet dust, meteorites, the chunks that land, are made of the denser, sturdier stuff of asteroids. Most asteroids are minding their own business in their own orbits, in the huge gap between Mars and Jupiter, about 120 million to 300 million miles from where you are reading this right now. Astronomers, being the creative folks they are, call this region the asteroid belt. But sometimes asteroids whack into each other, and hunks get bopped out of orbit. Some of those hunks head our way.

Between 10 and 50 meteorites crash down on us every day. Not to worry. Most are the size of small rocks. Seventy percent of them land in the ocean, which covers 70 percent of the surface of the earth. A majority of the rest fall in uninhabited areas where nobody notices.

Why wouldn’t we notice, you may ask, if the thing is glowing like a … well, like a shooting star?

It turns out that the glowing part only happens when the meteors are up higher. Meteoroids hit the upper atmosphere at incredibly high speeds after millions of miles of travel through the near-total vacuum of space. The friction with even the thin atmosphere at altitudes of 50 to 75 miles produces the flashes of light we see as the meteors incinerate. But the friction also slows them down. By the time they get down to 9 to 12 miles above the surface — if they’ve survived that far — they’ve slowed down enough, and cooled down enough, so we no longer see them. At that altitude they’re falling at about 200 to 400 mph.

Look out for the big one
Big meteors, above 8 tons, can maintain some of their extraterrestrial velocity. Really big ones, 1,000 tons and up, will come flying in with 75 percent of the speed they had in space. This would be a bad thing! Ask the dinosaurs. The meteorite that 65 million years ago hit the ocean and made the 110-mile-wide crater now buried under Chicxulub, Mexico, probably weighed as much as a trillion tons. It caused tidal waves that washed up to 430 miles inland around the entire Atlantic coast, forest fires all over the globe, and threw up so much dust that the “meteor winter” that lasted for years wiped out 70 percent of all the species on the planet.

Fortunately, the people who spend their time figuring such things out tell us that such collisions happen only once every 10 million years, give or take a few decades.

But twice in just the past hundred years big bolides have come flying in, exploding with thousands of times more force than the Hiroshima atomic bomb. One, about 200 feet across, exploded 5 miles up and still leveled 1,200 square miles of Siberian forest around Tunguska in 1908. Another one destroyed dozens of square miles of Amazonian jungle in 1930. Experts figure that a Tunguska-like bolide is a once-in-a-millennium occurrence.

A picture taken in the wake of the Tunguska Meteorite explosion, which occurred over Siberia in 1908, shows the devastation that even a "small" asteroid or comet can produce.

Your normal run-of-the-mill meteorite, though, is no bigger than your average earth rock. And after it’s been exposed to the weather, it’ll look like normal rocks, too, since meteorites are made of the same kinds of minerals, like olivine, pyroxene and feldspar, and metals like iron and nickel, that most terrestrial rocks are made of. If you look closely, and know what you’re looking for, you’ll see that meteorites have unique shapes and structures.

Of course, then there are the really big ones, like the Hoba meteorite, a 60-ton chunk of ex-asteroid found in South Africa in 1920. Or the “Iron Mountain” meteorite in Greenland hauled out by Robert Peary in 1984. The biggest chunk of the three pieces hauled out, now on display at the American Museum of Natural History in New York, weighed 34 tons.

The Iron Mountain meteorite allowed Inuits to survive in Greenland, which had no natural mineral deposits. European explorers in the early 1800s found the Inuits well-equipped with iron tools, knife blades, harpoon points and engraving tools. All from meteorites. Tools from outer space! In fact, archaeologists think that humans got the idea to use heat to smelt iron, moving man from the Bronze Age to the Iron Age, from the iron-rich meteorites they collected and turned into tools and weapons. The Hittites and Sumerians called iron “fire from heaven.” The Egyptian word for iron was “thunderbolt from heaven.”

Even in our jaded Western culture, the celestial fireworks can be deeply inspiring: If you go out in the pre-dawn hours and look up at the darkest part of the heavens, you could see what Coleridge was probably describing in “The Rime of the Ancient Mariner”:

The upper air burst into life!
And a hundred fire-flags sheen,
To and fro they were hurried about!
And to and fro, and in and out,
The wan stars danced between.

David Ropeik is a longtime science journalist and former director of risk communication at the Harvard Center for Risk Analysis. This article is an updated version of a report first published in August 2001.